mount is needed. Water makes up
two-thirds of the body, and nearly two quarts are given off daily in
the various excretions and secretions. If enough be not taken the
tissues get dry, and Nature indicates her want in thirst.

Another of these substances is _starch_, or its equivalent, sugar.
Rice, bread, and vegetables in general, are largely made up of this
starchy or sugary substance, which, as it contains a considerable
quantity of carbon, we speak of as the _carbonaceous_ element in food.
This is the substance which goes to feed the muscles, replacing the
waste from work done, just as fuel is required for the fires of an
engine.

Yet another substance in food is _fat_. It may be animal, such as beef
or mutton fat, and butter, or vegetable, as the oils in nuts, in the
olive, etc. Fat, like carbonaceous food, also goes to feed the muscles,
but both are required in a healthy diet.

Of the first importance, however, is the _proteid_ element in food.
Meat, milk, cheese, eggs, peas, etc., contain proteid in considerable
quantities. Its use is to repair the exhausted tissues themselves. The
muscles and nerves get worn out in their daily work, and require
rebuilding. This is what proteid goes to do, and from this, its high
import in animal economy, is called Proteid (protos--first). Finally,
in all natural foods there are certain _salts_, which also build up,
_e.g._, lime, which goes to make up bone. These salts may be seen in
the ash of any common vegetable after being burnt.

These four kinds of food substance make up our daily food, and a
certain amount of each substance is required to replace the daily
expenditure, a proportion which varies, however, under different
circumstances. _See_ Food in Health.

As the relative amount of proteid, carbonaceous matter, water, and
salts, may vary considerably in different articles, we rightly have
combinations of food at our meals. A pudding of corn-flour and water
contains no building material, hence we add milk and eggs, which do. A
meal of meat and cheese requires bread and potatoes, etc., etc.

Appetite is a good test of the amount and also of the particular kind
of food required, provided the appetite is in a healthy condition. If a
healthy man refrain from carbonaceous foods for a day or so, he feels a
great longing for them, a sign that the body really needs them. It is
of immense importance, then, that the appetite should not be accustomed
to over-indulgence, for the